1. Field of the Invention
The present invention relates in general to communication systems and specifically to a method and apparatus for improving the dynamic range of a receiver.
2. Description of the Related Art
Presently, various methods exist for receiving a radio signal. A popular method involves using a superheterodyne receiver. Superheterodyne receivers shift an incoming input signal to at least one intermediate frequency before producing the demodulated output signal. Filtering spurious signals and other unwanted signals can be accomplished more easily at the intermediate frequency (IF) than at baseband or at the carrier frequency at which the radio signal is transmitted. For receivers having a relatively small tuning range relative to the lowest carrier signal that is to be received, the IF is typically lower than the lowest carrier frequency and is chosen to minimize unwanted spurious signals.
Receivers having a wide tuning range (where the highest carrier frequency is many times the lowest carrier frequency) typically shift the incoming signal to an IF that is higher than the highest carrier frequency. The local oscillator (LO) that is used to shift the incoming signal to the IF must, therefore, tune from the IF to almost twice the IF to shift the incoming signal to the IF. The frequency synthesizer used to perform the LO function must be able to tune to high frequencies while maintaining a low phase noise. As a result, receivers using this topology typically exhibit LOs with substantially higher phase noise for carrier frequencies at the lower end of the tuning range of the receiver than other architectures.
One attempt at eliminating the requirement of the LO to produce signals with frequencies higher the carrier frequencies includes the use of a homodyne receiver. Homodyne receivers are also referred to as "zero IF receivers" and "zero-frequency IF receivers" since the tuned channel (incoming signal) is converted directly to baseband (D.C.). In a homodyne receiver, the LO is set to the nominal carrier frequency reducing the need for high frequency LOs. Typically, the LO signal is passed through a quadrature splitter that separates the LO signal into two signals having phases separated by 90 degrees. The input signal is split and mixed with the two LO signals producing a signal with an in-phase component, typically referred to as the "I channel", and a quadrature component typically referred to as the "Q channel". The I and Q channels represent the real part and imaginary part of a complex function of time, respectively. The I and Q channels are further processed to produce the desired output signal.
Homodyne receivers are limited, however, in dynamic range. Non-ideal characteristics of circuits such as the mixers, quadrature splitters and other circuits result in self interference with the homodyne receiver. Currently, in order to achieve a 40 dB signal-to-noise ratio (SNR), the dc offset in the mixers must be less than 1% of the signal amplitude, the error in the quadrature angle must be less than .+-.1 degree, and the disparity in gain between the I and Q channels must be less than 0.1 dB. Typical homodyne receivers have a practical SNR of 25-30 dB.
Methods have been developed in an attempt to reduce the effects of the non-ideal characteristics of the circuits for applications requiring greater dynamic range. At least one of the methods corrects for circuit errors by observing statistical properties of the input signal and subtracting an appropriate fraction of the Q channel from the I channel to correct for phase errors and scaling the Q channel in order to minimize the amplitude difference between the I and Q channels. These methods are limited, however, in that the DC (direct current) signal produced by the homodyne architecture can not be eliminated unless the modulated signal has a zero mean. The dynamic range, therefore, of the receiver is limited.
Further, these schemes are based on statistical properties of the signal and their correction involves feedback loops with long time constants needed for averaging signals. Therefore, the schemes are further limited in their ability to correct deficiencies quickly. In order to obtain an accurate average, slower circuits must be used.
Therefore, there exists a need for a method and apparatus for reducing phase and gain errors, reducing the DC signal components and providing a receiver with improved dynamic range.